At its heart the CGC really seems to be aiming to reduce the load on teachers to free them up to focus more time on designing good learning experiences, differentiation, and interacting with students more. However, these simplifications will require schools to jettison much of what they do, and for some that may be too bold and adventurous.
From an IB perspective I feel that the CGC and the PYP and MYP are incompatible. Not because they are mutually exclusive, but because of the constraints of the IB programmes. If you buy into the simplified and streamlined vision of CGC, then I can't imagine you would want to do the IB programmes.
It remains to be seen if the CGC will be robust enough to draw schools to its vision.